feat(observability): ship Grafana dashboard bundle

[however-yir]

Problem

Operators need ready-made dashboards to validate API latency, ingestion health, retrieval quality signals, and error rates after deployment.

Scope

• Add Grafana dashboard JSON files for API, ingestion, RAG, and JVM/runtime views.
• Document how dashboards map to Prometheus/Loki/Tempo signals.
• Link dashboards from the operations guide.

Acceptance criteria

• Dashboard JSON files are committed under observability/.
• docs/operations.md explains import and expected panels.
• README links the dashboard bundle.